US 7999544 B2 Abstract An MRI includes at least a singular imaging channel which uses a smoothly varying sampling pattern to sparsely sample k-space as a series of parallel lines over time to acquire data of an object. The MRI includes a computer in communication with the imaging channel that performs a signal processing routine to interpolate the data and fill in points of k-space which were not sampled by the imaging channels and produce an image of the object. A method for using an MRI includes the steps of acquiring data of an object with at least a singular imaging channel of the MRI which uses a smoothly varying sampling pattern to sparsely sample k-space as a series of parallel lines over time. There is the step of performing a signal processing routine with a computer in communication with the imaging channel to interpolate the data and fill in points of k-space which were not sampled by the imaging channels. There is the step of producing an image of the object.
Claims(8) 1. An MRI comprising:
at least a singular imaging channel which uses a smoothly varying sampling pattern to sparsely sample k-space as a series of parallel lines over time to acquire data of an object; and a computer in communication with the imaging channel that performs a signal processing routine to interpolate the data and fill in points of k-space which were not sampled by the imaging channels and produce an image of the object.
2. An MRI as described in
3. An MRI as described in
4. An MRI as described in
5. A method for using an MRI comprising the steps of:
acquiring data of an object with at least a singular imaging channel of the MRI which uses a smoothly varying sampling pattern to sparsely sample k-space as a series of parallel lines over time;
performing a signal processing routine with a computer in communication with the imaging channel to interpolate the data and fill in points of k-space which were not sampled by the imaging channels; and
producing an image of the object.
6. A method as described in
performing the signal processing routine with the computer in communication with the imaging channel to interpolate the data and fill in points of k-space which were not sampled by the imaging channel.
7. A method as described in
8. A method as described in
Description This application is a divisional of application Ser. No. 11/786,685 filed on Apr. 11, 2007 now U.S. Pat. No. 7,541,808. The present invention is related to producing an image of an object by using a smoothly varying sampling pattern to sparsely sample k-space as a series of parallel lines over time to acquire data of the object. More specifically, the present invention is related to producing an image of an object by using a smoothly varying sampling pattern to sparsely sample k-space as a series of parallel lines over time to acquire data of the object where the sparse sampling factor of the sampling pattern varies in a progressively increasing manner from the center of k-space to the periphery. There is a rapid imaging approach termed BRISK, see In terms of the sampling pattern, KT-BLAST employs a uniform sampling pattern, and employs a different reconstruction technique, which involves the use of “training data”, i.e. data in which the central region of k-space is sampled for 10-20 seconds, either separately from the KT-BLAST acquisition, or interleaved with the KT-BLAST acquisition. BRISK is characterized by a variable sampling scheme, but this sampling scheme has distinct sample rates applied to regions of k-space. In MACH, the variable sampling pattern is applied in a smoothly varying manner. The present invention pertains to an MRI. The MRI comprises at least a singular imaging channel which uses a smoothly varying sampling pattern to sparsely sample k-space as a series of parallel lines over time to acquire data of an object. The MRI comprises a computer in communication with the imaging channel that performs a signal processing routine to interpolate the data and fill in points of k-space which were not sampled by the imaging channels and produce an image of the object. The present invention pertains to a method for using an MRI. The method comprises the steps of acquiring data of an object with at least a singular imaging channel of the MRI which uses a smoothly varying sampling pattern to sparsely sample k-space as a series of parallel lines over time. There is the step of performing a signal processing routine with a computer in communication with the imaging channel to interpolate the data and fill in points of k-space which were not sampled by the imaging channels. There is the step of producing an image of the object. In the accompanying drawings, the preferred embodiment of the invention and preferred methods of practicing the invention are illustrated in which: Referring now to the drawings wherein like reference numerals refer to similar or identical parts throughout the several views, and more specifically to Preferably, the MRI Preferably, the sparse sampling factor of the sampling pattern varies in a progressively increasing manner from the center of k-space to the periphery. The progressively increasing sparse sampling pattern preferably smoothly varies such that the first-order derivative of the sparse sampling factor plotted from the center of k-space to the periphery does not have any discontinuities. Preferably, the computer The present invention pertains to a method for using an MRI Preferably, the acquiring step includes the step of acquiring data of the object with at least a second imaging channel The acquiring step preferably includes the step of acquiring data of the object with the imaging channels where the sparse sampling factor of the sampling pattern varies in a progressively increasing manner from the center of k-space to the periphery. Preferably, the acquiring step includes the step of acquiring data of the object with the imaging channels where the progressively increasing sparse sampling pattern smoothly varies such that the first-order derivative of the sparse sampling factor plotted from the center of k-space to the periphery does not have any discontinuities. The performing step preferably includes the step of redistributing with the computer The performing step preferably includes the step of interpolating the sparsely sampled data along a temporal dimension to populate data points that were not directly acquired by the imaging channels. In the operation of the invention, there is described an approach to acquire dynamic MRI In MACH, the sparse sampling factor varies in small increments from the center of k-space to the periphery; e.g. at the center of k-space data are sampled at every time point, at the next line, data are sampled using a sparse factor of 1.1, the next line uses a sparse factor of 1.2, etc., until the outermost line of k-space employs a sparse factor of 3.2. Based on this simple premise, several refinements were developed to produce the MACH sampling pattern shown in The MACH sampling and interpolation approach is suited for obtaining time resolved MRI The key features of the MACH scheme are: -
- 1. Its use of variable sparse sampling strategy as opposed to a uniform sparse sampling strategy.
- 2. The sparse sampling factor increases in a smooth manner from the center of k-space to the periphery. This results in a highly irregular appearance to the sparse sampling pattern, whereas most approaches (including BRISK and KT-BLAST) use highly regular and structured sampling.
- 3. Approaches that use irregular sampling patterns have tended to use random or pseudo random approaches, whereas the MACH approach is based on a progression of rates.
- 4. MACH does not need any training data, and the technique is self-contained, with the reconstruction process being applied uniformly irrespective of the dynamic structure being observed.
- 5. Since MACH images are generated from a full set of k-space data, the signal to noise ratio is inherently high. This allows the MACH approach to be combined with other rapid imaging strategies to produce images in a faster mode. This is particularly important for real-time monitoring of interventions that can be applied in the MRI
**10**scanner. Currently real-time imaging approaches in MRI**10**suffer from low temporal and spatial resolution. The MACH approach would allow higher resolution images to be acquired with a rapid update rate.
The present invention acquires time resolved magnetic resonance imaging data in a sparse manner such that the sparse-sampling factor varies in a progressively increasing manner from the center of k-space. The sparsely sampled data are interpolated along the temporal dimension to populate data points that were not directly acquired. The key features are 1) that the sparse sampling factor increases with distance from the center of k-space, and 2) that the sparse sampling factor varies in a smoothly continuous manner. The sparse sampling pattern can be achieved by a number of algorithms that achieve the two key features of 1) applying an increasing sparse factor from the center of k-space outwards and 2) increasing the sparse factor in a smoothly progressive manner. An illustrative example is provided here for k-space data comprised of a square matrix of size 256×256 to be acquired in a symmetric manner at regular time intervals. In this example the sparse sampling factor is set at 1 for the center of k-space and it is incremented in steps of 0.1 for successive line positions towards the upper and lower boundaries of k-space. In this concrete example, the sampling pattern is as follows: -
- The two central lines of k-space are sampled at each of the pre-determined time intervals, i.e. sparse sampling factor of 1
- The two lines contiguous with the central lines are sampled at a sampling rate of 1:1.1 frames per time interval, i.e. sparse sampling factor 1.1.
- The next two outermost lines of k-space are sampled at a rate of 1:1.2 frames per time interval, i.e. sparse sampling factor 1.2.
- The pattern is continued until the outermost lines of k-space are sampled at rate of 1:13.7 frames per time interval, i.e. sparse sampling factor 13.7.
For the above example, a graph showing the smoothly incrementing sparse sampling factor is illustrated in The formula used to determine the number of sampled points (NSP) sampled for an acquisition with a basic sample rate of R frames per second and a given sparse sampling factor (SSF) is given by the formula:
For example, for SSF=1.3, the number of sampled points is 76. In this case SPN ranges from 1 to 76, and the relative time points at which data are sampled are 1, 2, 3, 5, 6, 7, 8, . . . 98, yielding a total of 76 points. When using the above formulas to determine the sampled points relative to time position -
- Calculate the sample points by projecting the MACH sampling scheme over several cycles. For example when 3 cycles are chosen, if 100 time frames are to be acquired per cycle, then calculate the sampling scheme for time points ranging from 301 to 400.
- Select the MACH sampling pattern for the last half of the cycle calculated above (e.g. time points 351 to 400).
- Generate the mirror image of the half sampling pattern to define the first half of the sample pattern (e.g. sample points are to be based on the pattern of time points 400-351 followed by 351-400), such that where the two halves meet, the sample pattern is identical, as indicated in
FIG. 2 .
The sampling scheme outlined in -
- Redistribute sample points from time positions that have more than the average number of points to the same k-space position at the nearest time point that has fewer than the average number of sampled points.
The MACH sampling pattern is used to guide the acquisition of the phase-encoded lines of k-space for a time resolved acquisition. The MACH sampling pattern can be applied directly to any MRI For a given k-space line position, the records of the MACH sampling pattern are used to extract corresponding k-space lines for each time point. For each point of each k-space line, the time series of points are subjected to an interpolation procedure to generate k-space data corresponding to points that were not sampled, as indicated in -
- For a given k-space line, the sampled data are extracted and the time points at which they were acquired are noted.
- In the case of data that are expected to be cyclic, such as cardiac motion, the sampled period over which the data are captured is duplicated and positioned prior to the sampled points and following the sampled points. The corresponding time points are accordingly projected backwards or forwards in time respectively, and this information is stored.
- In the case that data are not expected to be cyclic for following treatment can be performed: if data for the first time point are not sampled, data from the first sampled point are duplicated backwards to populate each time point all the way back to the first time point; similarly, if data for the last time point are not sampled, data from the last sampled point are duplicated forward to populate each time point all the way to the last time point.
- For the extended multiple cycle data or the single cycle data amended as above, data interpolation is performed: either between the extended time points over three cycles (for periodic data) or between the first and last time points of the single cycle (for non-periodic data). Interpolation can be performed by a number of means including spline or cubic interpolation.
- A combination of interpolated and acquired data are extracted for time points ranging from the first point to the last point, such that data for each intermediate time point are represented.
A block diagram showing the key steps in implementing MACH are shown in Previously, a related rapid imaging approach termed BRISK was described. The present invention and BRISK are similar, but there are important differences. In the previously described BRISK approach, the sampling of k-space data over the time is performed in a sparse manner (i.e. data sampling is distributed over the so called k-t domain, i.e. k-space-time domain). A key feature of BRISK is that several sparse sampling factors are employed, with the lowest factor applied towards the center of k-space and higher factors applied towards the periphery of k-space, Since BRISK was introduced, others have introduced k-t sampling approaches, such as KT-BLAST, and SLAM. In general, these approaches employ a single sparse sampling factor that is uniformly applied for all k-space lines. Since a uniform sparse sampling factor is applied, there are no sudden transitions between k-space regions and thus Gibbs ringing is not a dominant artifact in these images. These techniques work best when the dynamic imaging region is restricted to 1/n of the field of view, where n is the sparse sampling factor. Under conditions where the dynamic region exceeds this extent, this class of approach is dominated by temporal blurring of each time frame. The techniques are estimated to perform at a level 50% less efficiently than BRISK, making them 100% less efficient than MACH. Although the invention has been described in detail in the foregoing embodiments for the purpose of illustration, it is to be understood that such detail is solely for that purpose and that variations can be made therein by those skilled in the art without departing from the spirit and scope of the invention except as it may be described by the following claims. The following references are incorporated by reference herein. KT-BLAST
- Tsao, J, Boesiger, P, Pruessmann, K, KT-BLAST and KT-SENSE: Dynamic MRI with High Frame Rate Exploiting Spatiotemporal Correlations; Magn. Reson. In Med. 50:1031-1042, 1003
BRISK - Doyle, M., Walsh, E. G., Foster, R. E., Pohost, G. M. “Rapid Cardiac Imaging with Turbo BRISK” Magn. Reson. in Med 1997; 37:410-417.
SLAM - Wolfgang G. Rehwald, PhD, Raymond J. Kim, MD, Orlando P. Simonetti, PhD, Gerhard Laub, PhD and Robert M. Judd, PhD Theory of High-Speed MR Imaging of the Human Heart with the Selective Line Acquisition Mode Radiology. 2001; 220:540-547.
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